The present invention relates to the field of dynamic gain controllers/equalizers, and more particularly, to methods of controlling optical characteristics of individual pixelated elements in a dynamic gain equalizer having an array of controllable pixelated elements used in optical transmission systems.
Optical transmission systems employ wavelength division multiplexing (WDM) to maximize use of a given band of wavelengths. The wavelength band is subdivided into multiple wavelengths or channels with each channel being independently modulated with a digital signal. Typically, the channels are multiplexed into an optical fiber and propagated along a transmission route between end systems.
For transmission paths greater than a certain distance it is necessary to incorporate optical filters at points along the route to compensate for wavelength-dependent system gain and attenuation. Optical fibers and optical amplifiers do not propagate all wavelengths with equal gain and attenuation and in many applications it is necessary to equalize the net gain and attenuation between channels. This can be done utilizing a dynamic gain equalizer (DGE) or gain controller.
Various dynamic gain controllers have been proposed that are based primarily on dispersing input light across an active element, or array of elements, that provide different attenuations in different spectral regions. The light that has been modified by the active element(s) is then undispersed and coupled to an output fiber. Ideally such controllers should be capable of providing a wide range of smoothly varying (low ripple) spectral attenuation functions to compensate for the spectral dependence of gain and attenuation, without distorting the various signal channels.
U.S. Pat. No. 5,805,759 issued Sep. 8, 1998 to Fukushima discloses a system that employs an attenuation plate with a spatially varying attenuation, which is movable in directions perpendicular to the propagation direction of the spectrally-dispersed light, thus varying the attenuation seen by various spectral regions. The Fukushima system is capable of providing a smoothly varying spectral attenuation function, but a given unit can provide only a limited set of such functions. Further, it is not possible to simply adjust the attenuation in a given spectral region using the Fukushima system.
U.S. Pat. No. 5,933,270 issued Aug. 3, 1999 to Toyohara discloses a system that employs a wavelength division multiplexer to couple light in different wavelength bands to different fibers, each of which is equipped with a variable light reflecting means. The Toyohara system is capable of providing a wide range of spectral attenuation functions. However, to avoid interference effects, it is necessary to insure that the wavelength bands do not overlap. As a result, the Toyohara system is not capable of providing a smoothly varying spectral attenuation function.
In an article titled xe2x80x9cLiquid-Crystal-Based Wavelength Selectable Cross-Connectxe2x80x9d by Ranalli et al. presented at ECOC 1999 (25th European Conference on Optical Communication), pages 68-9 vol. 1, published by the Societe des Electriciens et des Electroniciens (SEE) a liquid crystal based wavelength selective cross connect system is described. The Ranalli system employs a pixelated equalizer that uses an array of liquid crystal cells. However, similar to the Toyohara system, the Ranalli system is designed such that the wavelength bands do not overlap and, therefore, is not capable of providing a smoothly varying spectral attenuation function.
In summary, to provide a wide range of spectral attenuation functions, while maintaining a smoothly varying spectral attenuation function, and to provide attenuation adjustability in a given spectral region, it is desirable to use designs where each spectral region is acted upon by more than one of the individual attenuation elements. Applicant""s co-pending U.S. application Ser. No. 09/727,446 filed Dec. 4, 2000 describes an example of a dynamic gain flattening filter/dynamic gain equalizer, which discusses the need to have interactions with pixels in more detail.
However, for such equalizers, adjusting one of the attenuation elements will also change the attenuation for adjacent spectral bands, making it difficult to determine the correct settings for the attenuation elements to achieve a desired spectral attenuation function. Prior art in the field of gain controllers has not taught how to control such equalizers to accurately achieve desired smoothly varying attenuation functions without requiring multiple complex calculations and multiple iterations.
In accordance with one aspect of the present invention there is provided in a system having an array of pixelated elements for controlling incident light thereon, and having a processor for processing control information related to response transmission characteristics of the array of pixelated elements, a method of controlling characteristics of the array of pixelated elements in response to input data comprising the steps of: acquiring calibration information related to the response characteristics of the array of pixelated elements; acquiring a target response function for the array of pixelated elements; converting the target response function into a set of pixel amplitude field linear equations; and determining a set of input data values for controlling the array of pixelated elements based on the calibration information and the pixel amplitude field linear equations.
In accordance with another aspect of the present invention there is provided a method of controlling the optical characteristics of individual pixelated elements in a dynamic gain equalizer having an array of controllable pixelated elements, and having a processor for processing control information related to a response function of the dynamic gain equalizer, said method comprising the steps of: acquiring a predetermined target response function for the array of pixelated elements from a system controller; calculating initial attenuation settings for individual pixels of the array of pixelated elements based on stored data; calculating spectral attenuation that would result from the initial attenuation settings for individual pixels and determining deviations between the initial settings and settings to satisfy said target response; calculating input data for use by said processor to control said individual pixels if said deviations are within set limits; and sending said input data to said processor.
In accordance with another aspect of the present invention there is provided a computer program product for a system having an array of pixelated elements for controlling incident light thereon, and having a processor for processing control information related to response transmission characteristics of the array of pixelated elements, the computer program product comprising computer readable program code devices for controlling characteristics of the array of pixelated elements in response to input data comprising: acquiring calibration information related to the response characteristics of the array of pixelated elements; acquiring a target response function for the array of pixelated elements; converting the target response function into a set of pixel amplitude field linear equations; and determining a set of input data values for controlling the array of pixelated elements based on the calibration information and the pixel amplitude field linear equations.
In accordance with another aspect of the present invention there is provided a computer program product for controlling the optical characteristics of individual pixelated elements in a dynamic gain equalizer having an array of controllable pixelated elements, and having a processor for processing control information related to a response function of the dynamic gain equalizer, the computer program product comprising computer readable program code devices for: acquiring a predetermined target response function for the array of pixelated elements from a system controller; calculating initial attenuation settings for individual pixels of the array of pixelated elements based on stored data; calculating spectral attenuation that would result from the initial attenuation settings for individual pixels and determining deviations between the initial settings and settings to satisfy said target response; calculating input data for use by said processor to control said individual pixels if said deviations are within set limits: and sending said input data to said processor.
In accordance with an exemplary embodiment, the present invention provides a computationally efficient method for controlling the settings of individual pixels of a pixelated dynamic gain controller to achieve a desired spectral response function. In particular, the method of the present invention provides an accurate first estimate of the optimum settings and can be iterated without any external information to converge to the optimum settings. The settings can be adjusted to compensate for the effects of the edges of the pixel array.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.